In the printing of bar code labels, it is sometimes desirable or required to have a color image on the label as well as the typically black bar code image. The previous approach to providing printers having the capability of printing such labels has been to utilize a single printing technology in the printer. Since the various known printing technologies have widely differing characteristics in regard to the print quality produced, the use of a single technology to produce both color images and bar code images has required compromises that result in severe limitations on the print quality of the color image and/or the bar code image.
For bar codes and other machine readable symbologies, it is desirable that the printed image have sharp boundaries that are accurately detectable by a machine. Sharp boundaries are produced by pixels (printed dots) that have essentially the same dimensions-as the addressability of the print engine. In order words, the printed dots remain discrete and do not overlap or bleed into each other. The main technologies that meet this criterion are direct thermal and thermal transfer. For color images, color saturation is a major consideration for producing a high quality image. Pixels that are large relative to the addressability of the printer cause undesirable bleeding of bar code borders across narrow spaces in the symbol, significantly hurting readability. On the other hand, relatively large pixels provide the desired color saturation for color images. Technologies that produce relatively large pixels include ink jet, electrophotography, and dot matrix impact.
The different desirable characteristics of bar code and color images and the different technologies that produce these desirable characteristics have led to the compromises referred to above. The standard industry practice has been to use the sharp definition technologies of direct thermal and thermal transfer for the printing of bar codes, whether or not each printed label also includes a color image. Currently existing color bar code printers have generally used thermal transfer technology. The use of this technology has caused either high printer cost or low throughput, i.e. slow printing rates, as well as relatively high media expense. In order to maintain a relatively high throughput, color thermal transfer printers must essentially replicate the entire printing mechanism for each color desired. Therefore, the equipment cost for a color bar code printer that prints black as well as a normal range of colors is nearly four times that of a monochrome printer. One approach to achieving lower printer costs has been to sequentially array on a ribbon all of the colors to be used in the images to be produced by a printer. In such an arrangement, the media must be fed backward and then forward for each color after the first color. The backward and forward feeding causes slow throughput as well as registration inaccuracies. In both the arrangement of multiple print mechanisms and the arrangement of a sequentially arrayed ribbon, a print head lifting mechanism may be required to allow independent feeding of ribbon and paper. Print head lifting mechanisms contribute significantly to reliability problems, noise, and cost.
The failure of the industry to provide an acceptable color bar code printer is a result of a general failure to fully appreciate the nature of the problem and the actual needs of most situations in which bar code labels with color images are to be printed. This lack of understanding has led to the unsuccessful efforts described above. Such efforts have resulted in printer designs that are either excessively expensive or that fail to meet practical needs.